1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with a zoom lens; these may be suitable for a photographic optical system used in an image pickup apparatus, such as a video camera, an electronic still camera, a monitoring camera, a TV camera, and a silver-halide film camera.
2. Description of the Related Art
As the size of imaging apparatuses continues to decrease, a photographic optical system used for an image pickup apparatus is required to be equipped with a zoom lens that has short total lens length, small size, wide angle of view, high resolution, and high zoom ratio, among other significant characteristics.
In order to realize a high zoom ratio in a zoom lens, a positive-lead type zoom lens configuration is advantageous. In the positive-lead type lens configuration, a lens unit having positive refractive power is located at a position closest to the object to be imaged (i.e., at the object side of the zoom lens).
A four-unit zoom lens is known. The four-unit zoom lens includes, in order from an object side to an image side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power.
There is known a high-zoom-ratio zoom lens that performs zooming by moving all of the lens units of a four-unit zoom lens. U.S. Pat. No. 4,657,351 discusses a zoom lens that achieves compactness of the entire zoom lens by appropriately setting a focal length (reciprocal of refractive power) of each lens unit and appropriately defining a moving locus of a second lens unit during zooming.
Also, there is known a compact zoom lens that performs zooming by moving two lens units of a four-unit zoom lens. Japanese Patent Application Laid-Open No. 2006-171431 discusses a rear-focus type zoom lens that performs focusing while moving a second lens unit to perform variable magnification and moving a fourth lens unit to correct a variation on an image plane caused by variable magnification. In addition, an increase in an effective diameter of a front lens is reduced by using a high-refractive-index material in lenses constituting a first lens unit.
In order to realize a zoom lens having a wide angle of view, a high zoom ratio, and a small size in the entire zoom lens, the number of lenses is reduced while reinforcing refractive power of each lens unit constituting the zoom lens. However, if the refractive power of each lens surface is increased, a lens thickness is increased and various aberrations are increased. As a result, it is difficult to correct various aberrations.
Also, if it is intended to retract and store each lens unit when a camera is not in use, an influence of possible tilt of lenses and lens units may increase in terms of a mechanism structure. In this case, if the sensitivity to decentering of lenses and lens units is high, optical performance degrades and an image shake occurs during zooming, resulting in image degradation. Therefore, in the zoom lens, it is desirable to reduce the sensitivity to decentering of lenses or lens units as much as possible.
In a method of performing variable magnification (zooming) by moving only a second lens unit and a fourth lens unit of a four-unit zoom lens, most variable magnification is performed by the second lens unit. For this reason, it is customary to increase the refractive powers of the first and second lens units. However, when the refractive power of the second lens unit is increased, aberration variation tends to increase during zooming.
On the other hand, a method of moving all of the four lens units is suitable for a retractable structure because refractive power of a first lens unit or a second lens unit is reduced and sensitivity to decentering from an optical axis is reduced. In addition, when moving all of the four lens units, it is easy to reduce aberration variation during zooming. However, if it is intended to downsize the entire zoom lens while achieving a wider angle of view and a higher zoom ratio, various aberrations increase in a middle zoom area of the entire zoom range. Therefore, it is difficult to correct these aberrations.
In the zoom lens discussed in U.S. Pat. No. 4,657,351, the number of lenses of each lens unit is increased so as to obtain a desired optical performance. Therefore, even when a lens barrel is provided with a retractable structure, it is difficult to reduce a total lens length. The zoom lens discussed in Japanese Patent Application Laid-Open No. 2006-171431 achieves downsizing of the entire zoom lens while achieving a high zoom ratio. However, since principal variable magnification depends on the second lens unit, aberration variation tends to increase during zooming.
In order to achieve downsizing of the entire zoom lens, a wide angle of view, and a high zoom ratio and obtain high optical performance over an entire zoom range, it is important to appropriate set the lens configuration of each lens unit. In the four-unit zoom lens described above, in particular, it is important to appropriate set the lens configuration of the first lens unit and an amount of movement of the first lens unit according to zooming. If these configurations are not appropriately set, it is very difficult to achieve downsizing of the entire zoom lens, a wide angle of view, and a high zoom ratio and obtain high optical performance over an entire zoom range.